Marine trunk piston engines generally use Heavy Fuel Oil (‘HFO’) for offshore running. Heavy Fuel Oil is the heaviest fraction of petroleum distillate and comprises a complex mixture of molecules including up to 15% of asphaltenes, defined as the fraction of petroleum distillate that is insoluble in an excess of aliphatic hydrocarbon (e.g. heptane) but which is soluble in aromatic solvents (e.g. toluene). Asphaltenes can enter the engine lubricant as contaminants either via the cylinder or the fuel pumps and injectors, and asphaltene precipitation can then occur, manifested in ‘black paint’ or ‘black sludge’ in the engine. The presence of such carbonaceous deposits on a piston surface can act as an insulating layer which can result in the formation of cracks that then propagate through the piston. If a crack travels through the piston, hot combustion gases can enter the crankcase, possibly resulting in a crankcase explosion.
It is therefore highly desirable that trunk piston engine oils (‘TPEO’s) prevent or inhibit asphaltene precipitation. The prior art describes ways of doing this, including use of metal carboxylate detergents.
U.S. Pat. No. 7,053,027 describes use of one or more overbased metal carboxylate detergents in combination with an antiwear additive in a dispersant-free TPEO.
The problem of asphaltene precipitation is more acute at higher basestock saturate levels and WO 2008/128656 describes a solution by use of an overbased metal hydrocarbyl-substituted hydroxybenzoate detergent having a basicity index of less than 2 and a degree of carbonation of 80% or greater in a marine trunk piston engine lubricant to reduce asphaltene precipitation in the lubricant. Mentioned, but not exemplified, are lubricants comprising Group III and Group IV basestocks, and exemplified are lubricants comprising a Group II basestock, all of which basestocks have high saturates levels.
The art does not, however, concern itself with the influence of the diluent present in the metal carboxylate detergent. Although US-A-2007/0027057 describes alkylhydroxybenzoate additives made with a Group II diluent oil (see paragraph 0174), it is concerned with provision of a low sulphur content, not use in a TPEO to control asphaltene dispercancy.